outlet/flow: persist decoders' state

Currently, for NetFlow, persist data/options templates and options. This
should allow the outlet to restart without loosing any message.

Fix #2100

console/data/docs/02-configuration.md

@@ -608,7 +608,7 @@ exporter-classifiers:
 
 ### ClickHouse
 
-The ClickHouse component pushes data to ClickHouse. There are two settings that
+The ClickHouse component pushes data to ClickHouse. There are three settings that
 are configurable:
 
 - `maximum-batch-size` defines how many flows to send to ClickHouse in a single batch at most
@@ -621,6 +621,14 @@ send a batch of size at most `maximum-batch-size` at least every
 The default value is 100 000 and allows ClickHouse to handle incoming flows
 efficiently.
 
+### Flow
+
+The flow component decodes flows received from Kafka. There is only one setting:
+
+- `state-persist-file` defines the location of the file to save the state of the
+  flow decoders and read it back on startup. It is used to store IPFIX/NetFlow
+  templates and options.
+
 ## Orchestrator service
 
 The three main components of the orchestrator service are `schema`,

console/data/docs/99-changelog.md

@@ -22,6 +22,7 @@ identified with a specific icon:
 - 🩹 *outlet*: accept flows where interface names or descriptions are missing
 - 🩹 *docker*: update Traefik to 3.6.1 (for compatibility with Docker Engine 29)
 - 🌱 *common*: enable block and mutex profiling
+- 🌱 *outlet*: save IPFIX decoder state to a file to prevent discarding flows on start
 - 🌱 *config*: rename `verify` to `skip-verify` in TLS configurations for
   ClickHouse, Kafka and remote data sources (with inverted logic)
 - 🌱 *config*: remote data sources accept a specific TLS configuration